Method for controlling foam production in reduced pressure fining

ABSTRACT

A method for controlling the foam produced when a molten material encounters reduced pressure in a vacuum chamber includes passing the molten material through an aging zone in the vacuum chamber in which the molten material is allowed to drain from between the bubbles of the foam and then collapsing the bubbles of the drained foam.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The invention relates generally to reduced pressure fining, aprocess for removing trapped bubbles in molten material, e.g., moltenglass. More specifically, the invention relates to a method forcontrolling the foam produced when the molten material encountersreduced pressure in a reduced pressure finer.

[0003] 2. Background Art

[0004] In industrial glassmaking, a glass batch is made by mixing inblenders a variety of raw materials obtained from properly sized,cleaned, and treated materials that have been pre-analyzed for impurity.Recycled glass called cullet may also be mixed with the raw materials.For the most commonly produced soda-lime glass, these raw materialsinclude silica (SiO₂), soda (Na₂O), lime (CaO), and various otherchemical compounds. The soda serves as a flux to lower the temperatureat which the silica melts, and the lime acts as a stabilizer for thesilica. A typical soda-lime glass is composed of about seventy percentsilica, fifteen percent soda, and nine percent lime, with much smalleramounts of the various other chemical compounds. The glass batch isconveyed to a “doghouse”, which is a hopper at the back of the meltingchamber of a glass melting furnace. The glass batch may be lightlymoistened to discourage segregation of the ingredients by vibrations ofthe conveyor system or may be pressed into pellets or briquettes toimprove contact between the particles.

[0005] The glass batch is inserted into the melting chamber bymechanized shovels, screw conveyors, or blanket feeders. The heatrequired to melt the glass batch may be generated using natural gas,oil, or electricity. However, electric melting is by far the most energyefficient and clean method because it introduces the heat where neededand eliminates the problem of batch materials being carried away withthe flue gases. To ensure that the composition of the molten glass ishomogenous throughout, the molten glass is typically stirred together ina conditioning chamber that is equipped with mechanical mixers ornitrogen or air bubblers. The molten glass is then carried in a set ofnarrow channels, called forehearth, to the forming machines. In themelting chamber, large quantities of gas can be generated by thedecomposition of the raw materials in the batch. These gases, togetherwith trapped air, form bubbles in the molten glass. Large bubbles riseto the surface, but, especially as the glass becomes more viscous, smallbubbles are trapped in the molten glass in such numbers that theythreaten the quality of the final product. For products requiring highquality glass, e.g., optical lenses, television panels, and liquidcrystal displays, the trapped bubbles are removed from the molten glassprior to feeding the molten glass into the forming machines.

[0006] The process of removing bubbles from molten glass is calledfining. One method for fining glass involves adding various materialsknown as fining agents to the glass batch prior to mixing in theblenders. The primary purpose of the fining agents is to release gas inthe molten glass when the molten glass is at the proper finingtemperature. The released gas then diffuses into gas bubbles in themolten glass. As the bubbles become larger, their relative buoyancyincreases, causing them to rise to the surface of the molten glass wherethey are released. The speed at which the bubbles move through themolten glass may be increased by reducing the viscosity of the moltenglass, and the viscosity of the molten glass can be reduced byincreasing the temperature of the molten glass. An effective finingagent for atmospheric pressure, glass melting and fining processesshould be able to release a large amount of fining gases as thetemperature of the molten glass is increased to the temperature rangewhere the viscosity of the molten glass is sufficiently low, i.e., 1300°C. to 1500° C. for soda-lime glass. Examples of fining agents that aresuitable for use with soda-lime glass are arsenic oxide (As₂O₃) andantimony oxide (Sb₂O₃). These fining agents are, however, detrimental tothe environment and require careful handling.

[0007] Another method for fining glass involves passing the molten glassthrough a low pressure zone to cause the bubbles in the molten glass toexpand and rise quickly to the surface of the glass. This process istypically referred to as reduced pressure fining or vacuum fining. Thereare various configurations of reduced pressure finers. U.S. Pat. No.5,849,058 to Takeshita et al. discloses the general structure of asiphon-type reduced pressure finer. The reduced pressure finer, as shownin FIG. 1, includes a vacuum vessel 1 disposed in vacuum housing 2. Thevacuum vessel 1 has one end connected to an uprising pipe 3 and anotherend connected to a downfalling pipe 4. The uprising pipe 3 and thedownfalling pipe 4 are made of platinum, a material that can withstandthe high temperature of the molten glass and that is not easilycorroded. The vacuum vessel 1, the uprising pipe 3, and the downfallingpipe 4 are heated by electricity. An insulating material 5 is providedaround the vacuum vessel 1, the uprising pipe 3, and the downfallingpipe 4. Typically, the insulating material 5 consists generally ofinsulating bricks and doubles as a structural support for the uprisingpipe 3 and the downfalling pipe 4. The bottom ends of the uprising pipe3 and the downfalling pipe 4 that are not connected to the vacuum vessel1 extend through the vacuum housing 2 into the storage vessels 6 and 7,respectively. The storage vessel 1 is connected to receive molten glassfrom a glass melting furnace (not shown).

[0008] Flow of molten glass through the uprising pipe 3, the vacuumvessel 1, and the downfalling pipe 4 follows the siphon principle.Accordingly, the liquid surface of the molten glass in the vacuum vessel1 is higher than the liquid surface of the molten glass in the storagevessel 6, and the pressure in the vacuum vessel 1 is lower than thepressure in the storage vessel 6. The pressure in the vacuum vessel 1 isrelated to the elevation of the liquid surface of the molten glass inthe vacuum vessel 1 with respect to the liquid surface of the moltenglass in the storage vessel 6. The height of the liquid surface of themolten glass in vacuum vessel 1 with respect to the liquid surface ofthe molten glass in the storage vessel 6 is set based on the desiredfining pressure and the rate at which molten glass is flowing into thevacuum vessel 1. The molten glass with the trapped bubbles istransferred from the glass melting furnace (not shown) into the storagevessel 6. Because the pressure in the vacuum vessel 1 is less than thepressure in the storage vessel 6, the molten glass in the storage vessel6 rises through the uprising pipe 3 into the vacuum vessel 1. Thepressure in the vacuum vessel 1 is brought to reduced pressure conditionof less than the atmospheric pressure, typically {fraction (1/20)} to ⅓atmospheric pressure. As the molten glass passes through the vacuumvessel 1 and encounters the reduced pressure, the bubbles in the moltenglass expand and quickly rise to the surface of the molten glass. Therefined glass descends into the storage vessel 7 through the downfallingpipe 4.

[0009] Foam is produced in the headspace 8 as the molten glassencounters the reduced pressure in the vacuum vessel 1. The headspace 8must either be large enough to contain the foam, or the foam must becontrolled, to prevent equipment flooding and other process upsets andquality problems. In large scale processes, it is usually not practicalto make the headspace 8 big enough to contain the foam, especiallybecause the headspace 8 must be maintained airtight. U.S. Pat. No.4,704,153 issued to Schwenninger et al. discloses a method forcontrolling foam that includes providing a burner in the headspace.Schwenninger et al. in the '153 patent disclose that the heat from theburner reduces the viscosity of the foam and increases the volume of thebubbles of the foam, causing the bubbles to burst. U.S. Pat. No.4,794,860 issued to Welton discloses a foam control method that includesapplying agents to the foam, which cause coalescence of the bubblesand/or interrupt the surface tension in the bubble membranes so that thebubbles burst. Examples of foam breaking agents include water, alkalimetal compounds such as sodium hydroxide or sodium carbonate, alcohol,and fuel oil. U.S. Pat. No. 4,849,004 issued to Schwenninger et al.discloses a foam control method that includes suddenly changing thepressure in the headspace so as to disrupt the bubble membranes of thefoam, thereby bursting a substantial portion of the bubbles andexpediting collapse of the foam. A sudden surge of low pressure is usedto expand the foam bubbles beyond their limit of elasticity, at whichpoint they break. The pressure surges may be applied at intervals ofseveral minutes, and the duration of the pressure surges may be on theorder of a few seconds.

SUMMARY OF THE INVENTION

[0010] One aspect of the invention is a method for controlling the foamproduced when a molten material encounters reduced pressure in a vacuumchamber. The method includes passing the molten material through anaging zone in the vacuum chamber, wherein the molten material is allowedto drain from between the bubbles of the foam, and breaking the bubblesof the drained foam. In another aspect, the method includes allowing thefoam to flow from the vacuum chamber into a foam chamber separate fromthe vacuum chamber. In yet another aspect, the method includes allowingthe molten material to flow into the vacuum chamber through an inclinedconduit, wherein the bubbles in the molten material collect on a surfaceof the inclined conduit as the molten material flows through theinclined conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows the general structure of a siphon-type reducedpressure finer.

[0012]FIG. 2 is a schematic illustration of a reduced pressure finer.

[0013]FIG. 3 illustrates how foam density changes with time.

[0014]FIG. 4 illustrates how fragility of the foam increases with time.

[0015]FIG. 5A is a schematic illustration of a rotating cone.

[0016]FIG. 5B is a schematic illustration of a rotating bar.

[0017]FIG. 6 shows a gas jet impinging on a glass/foam surface.

[0018]FIGS. 7 and 8 show different embodiments of foam chambers.

[0019]FIGS. 9 and 10 show reduced pressure finers with inclined risers.

DETAILED DESCRIPTION

[0020]FIG. 2 is a schematic illustration of a siphon-type reducedpressure finer 10 suitable for removing bubbles trapped in molten glassor other molten material. There are other types of reduced pressurefiner configurations that may be suitable for use with the invention,see, for example, U.S. Pat. No. 4,780,122 issued to Schwenninger et al.which discloses a non-siphon-type reduced pressure finer. The reducedpressure finer 10 includes a vacuum housing 12 which has an inlet end 14and an outlet end 16. The vacuum housing 12 is maintained in asubstantially airtight condition by providing seals 20, 22 at the inletend 14 and the outlet end 16, respectively. A finer chamber 24 ismounted inside the vacuum housing 12. The finer chamber 24 is encased inrefractory insulation 25. The finer chamber 24 is usually heated bymeans of electricity, and, at least, the bottom portion 27 of the finerchamber 24 that comes in contact with the molten glass is lined with ormade of a material that has a high melting point and that is corrosionresistant, e.g., platinum or platinum alloy.

[0021] A riser tube 28 rises from the inlet end 14 of the vacuum housing12 to an inlet port 30 in the finer chamber 24, and a descender tube 32descends from an outlet port 34 in the finer chamber 24 to the outletend 16 of the vacuum housing 12. Typically, the tubes 28, 32 are made ofa material such as platinum or platinum alloy. Like the finer chamber24, the tubes 28, 32 are also heated. Refractory insulation 33, 35 areprovided around the tubes 28, 32 to minimize heat loss from the tubes28, 32 and provide structural support to the tubes 28, 32. The materialused to construct the tubes 28, 32 expands, but at a different rate thanthe refractory insulation 33, 35 around the tubes 28, 32. Thus,maintaining a reliable seal at the inlet end 14 and the outlet end 16,where the tubes 28, 32 exit the vacuum housing 12, is difficult. U.S.application Ser. No. 09/606,953, entitled “Tubing System for ReducedPressure Fining,” filed Jun. 29, 2000 by R. W. Palmquist, discloses asuitable method for sealing the inlet and outlet ends of the vacuumhousing while accommodating expansion of the tubes and the refractoryinsulation around the tubes.

[0022] In a typical glass fining process, the tubes 28, 32 and the finerchamber 24 are heated to a selected temperature, e.g., 1400° C. Moltenglass from a glass melting furnace 36 then flows into the riser tube 28through a conduit 38. A valve (not shown) may be provided to control therate at which molten glass is transferred into the riser tube 28, thusmaking it possible to control the pressure in the finer chamber 24. Astir chamber or storage vessel 42 that is connected to the outlet end 16of the vacuum housing 12 is also preheated to a selected temperature,e.g., 1400° C., and recycled glass, also known as cullet, is fed intothe stir chamber 42 and allowed to melt until the level of glass in thestir chamber 42 reaches the outlet end 44 of the descender tube 32. Oncethe outlet end 44 of the descender tube 32 is immersed in molten glass,the pressure in the finer chamber 24 is slowly reduced so that moltenglass is drawn into the finer chamber 24 through the tubes 28, 32. Thepressure in the finer chamber 24 may be reduced by using a vacuum pump(not shown) to draw air out of the finer chamber 24. While glass isdrawn into the finer chamber 24, more cullet may be melted in the stirchamber 42 to ensure that the outlet end 44 of the descender tube 32remains immersed in molten glass. Once the molten glass in the finerchamber 24 reaches the desired level, flow through the reduced pressurefiner 10 is started by drawing glass out of the stir chamber 42.

[0023] During operation, molten glass flows through the reduced pressurefiner 10 like a siphon. The pressure in the finer chamber 24 is reducedbelow atmospheric pressure to encourage expansion of the bubbles trappedin the molten glass. To achieve a desired sub-atmospheric pressure inthe finer chamber 24, the surface 46 of the glass in the finer chamber24 is elevated a predetermined height above the surface 48 of the glassin the glass melting furnace 36. When the molten glass enters the finerchamber 24 and encounters the reduced sub-atmospheric pressure in thefiner chamber 24, the trapped bubbles in the molten glass rapidly expandand move to the surface 46 of the glass. It is important to select anappropriate length for the finer chamber 24 that will allow adequateresidence time for the trapped bubbles in the glass to rise to the glasssurface 46 and break. It is also important that a headspace 50 above theglass surface 46 is provided to accommodate the layer of foam 45generated as a result of the rapidly expanding bubbles moving to theglass surface 46. The foam 45 created during the fining process ispersistent and can quickly occupy all available space if not controlled.A baffle plate 49 is positioned near the outlet port 34 of the finerchamber 24 to keep foam out of the descender tube 32.

[0024] The foam 45 that is newly created as the molten glass Gencounters the reduced pressure in the finer chamber 24 is resilient anddense. This newly created foam 45 may contain as much as 50% glass.However, as shown in FIGS. 3 and 4, the resiliency and density of thefoam 45 decreases with time as the foam 45 moves along the finer chamber24 and the glass drains from the foam back into the molten glass Gbeneath it. Old foam 45 may contain as little as 5-10% glass. As thefoam 45 becomes more fragile, it becomes easier to break the foam. Forexample, as demonstrated in FIG. 4, when the foam 45 is allowed to agefor 10 minutes, foam breakers can be applied to the foam 45 for aduration of about 2.5 minutes to obtain a reduction in foam height ofabout 0.25 inches. However, when the foam 45 is allowed to age for 30minutes, foam breakers can be applied to the foam 45 for the sameduration to obtain a reduction in foam height of about 2 inches, aseven-fold reduction in foam height for the same 2.5-minute foambreaking action. The invention includes providing a foam aging zone 52in the finer chamber 24 that allows the foam 45 to “age” or drain for acritical time before breaking the foam. Foam breakers 54 are thenapplied at the end of the foam aging zone 52 to break the foam 45. Thefoam breakers 54 are inserted through ports 55 in the finer chamber 24.

[0025] In one embodiment, the foam breakers 54 are mechanical rotators,e.g., the rotating cone 56 (shown in FIG. 5A) and the rotating bar 58(shown in FIG. 5B). These rotators stretch and rupture the foams. Therotating cone 56 (shown in FIG. 5A) includes an inverted cone 590 thatis mounted at the end of a rod 60. The angle α of the cone is steep,e.g., 70°, to minimize outward flinging of the glass, which would createbubbles anew when it hits the side walls of the finer chamber 24 andrecombines with the molten glass G. Because the influence of a rotatoris confined mainly to an area of the same diameter as the rotator,multiple or moving rotators are needed to treat larger or non-circularareas of foam. The number and placement of the rotators are determinedby the fining and foaming behavior of the glass and the amount ofshearing required to break the foam. Of course, based on the fining andfoaming behavior of the glass, the length of the aging zone 52 should beselected such that when the rotators are applied to the foam 45 at theend of the aging zone 52, the desired rapid breaking of the foam 45 isachieved.

[0026] The foam breakers 54 could also be nozzles which produce gasjets. FIG. 6 shows an example of a nozzle 63 that includes twoconcentric tubes 62, 64. The inner tube 62 is connected to a gas source(not shown), and the outer tube 64 is connected to an another gas source(not shown). The gases exit the nozzle 62, 64 in form of a jet 65, whichthen impinges on the foam 45. The impact force of the gas jet 65 as itimpinges on the foam 45 breaks the foam 45. In one embodiment, the gasflowing through the inner tube 62 is hydrogen, and the gas flowingthrough the outer tube 64 is oxygen. The flow rates of the hydrogen andoxygen gases are appropriately selected so that the ratio of hydrogen tooxygen is outside of a range in which stable flames are formed. Underoptimum conditions, the jet 65 mechanically breaks the foam 45 withoutlowering the foam temperature or heating the foam. Heating the foam 45would cause thermal reboil of the foam and create new bubbles and foam.It should be clear, however, that the nozzle 63 shown in FIG. 6 is justone example of a nozzle that may be used to produce a gas jet. Othersuitable nozzle configurations may be selected based on the desireddepth and area of penetration of the jet 65. Also, the gas (or gases)used in producing the gas jet 65 does not have to be combustible.Preferably, the gas (or gases) used in producing the gas jet 65 can bere-adsorbed into the glass when the molten glass G is later cooled downand returned to atmospheric pressure.

[0027] The gas jet 65 may be applied to the foam 45 in pulses to reducethe amount of gas required to produce the jet and to extend the area ofpenetration of the jet. That is, instead of continuously supplying thefuel and oxygen to the nozzle 63, the fuel and oxygen can be supplied tothe nozzle 63 at predetermined intervals. These predetermined intervalsmay have varying lengths depending on the rate at which the foam isproduced and the fragility of the foam at the area of penetration of thejet. Alternatively, multiple nozzles can be positioned across the widthand down the length of the foam surface to distribute the breakingforce. The multiple nozzles may produce pulsed jets. The pulses can begenerated at different times in a pattern that will break the foam andprevent the foam from circumventing a single jet area. The nozzles mayalso be rotated and moved across the foam. Alternatively, the foambreakers 54 may be gas sprays in which particles of materials such asmolybednum, silica, or cullet particles are dispersed. The more massiveliquid/solid particles in the gas sprays would impart a higher impactforce to break the foams than the particle-free gas jets. The materialsdispersed in the gas sprays are selected to be readily incorporated inthe glass without producing bubbles or other defects.

[0028]FIG. 7 illustrates another method for controlling the volume offoam 45 in the headspace 50. The method includes allowing the foam 45 inthe headspace 50 to flow into a separate foam chamber 68, which isdisposed within a vacuum housing 70 and maintained at the same pressureas the finer chamber 24. The glass between the bubbles of the foam 72 inthe foam chamber 68 drains through an outlet port 74 in the foam chamber68. A foam breaking device may be employed at the outlet end 76 of thefoam chamber 68 to break the foam 72 after the foam has adequately“aged” or drained. Any of the previously discussed foam breakers 54(shown in FIGS. 5A, 5B, and 6) may be used. It should be noted that theglass surface 78 in the foam chamber 52 will be about the same as theglass surface 46 in the finer chamber 24. The glass draining through theoutlet port 74 can be directed back into the riser tube 28, or may bedirected into a storage vessel (not shown) and later recycled throughthe glass melting furnace (not shown) as cullet. To avoid large capitalexpenditures, the foam chamber 68 may be made out of atemperature-resistant refractory such as alumina instead of a preciousmetal such as platinum.

[0029] Instead of allowing the glass between the bubbles of the foam 72to drain through the outlet port 74, the glass can be cooled andcrushed. The crushed glass can be recycled through the glass meltingfurnace (not shown) or used in any application that calls for cullet.FIG. 8 shows a foam chamber 80 that can be used to cool and crush glass.The foam chamber 80 has an inlet port 82 for receiving foam from thefiner chamber 24. A set of chilled rollers 84 is mounted at the uppersection 86 of the foam chamber 80. The chilled rollers 84 are providedto cool and crush the foam 72 received at the inlet port 82. The finegrained cullet 88, i.e., crushed glass, produced by the chilled rollers84 settles to the bottom of the foam chamber 80. The cullet 88 may beperiodically collected by opening a gate 90 at the bottom of the foamchamber 80. Before opening the gate 90, another gate 92 above the gate90 is closed to keep the cullet that is being produced by the chilledrollers 84 from dropping to the bottom of the foam chamber 80. The gate92 is again opened and the gate 90 is closed after collecting the cullet88. A purge gas 94 may be blown into the conduit 96, which connects thefiner chamber 24 to the foam chamber 80, to assist in moving the foam 72into the foam chamber 80. An outlet port 98 is provided in the foamchamber 80 through which the purge gas 94 may exit the foam chamber 80.

[0030]FIG. 8 illustrates another method for controlling the volume offoam 45 in the headspace 50. The method includes inclining the risertube 28 with respect to the vertical so that a separation of bubblesfrom the molten glass occurs as the molten glass flows into the finerchamber 24. This allows the glass to be delivered to the finer chamber24 relatively free of bubbles. In addition, the bubbles, because oftheir collection at the top 100 of the riser tube 28, have had anincreased opportunity to coalesce. This increased bubble size aids thebreaking of the foam 45 in the finer chamber 24. As shown in FIG. 9, astandpipe 102 can be added to the riser tube 28 to remove the bubbles atthe top 100 of the riser tube 28 completely from the molten glass. Thestandpipe 102 is placed so that it rises above the glass line 46 in thefiner chamber 24. The atmosphere over the standpipe 102 is at the samepressure as the finer chamber 24. Alternatively, the standpipe 102 couldbe placed at a lower elevation, that is, closer to the inlet end of theriser tube 28. In this case, the glass line in the standpipe 102 wouldbe below the glass line 46 in the finer chamber 24. This would requirethat the atmosphere over the standpipe 102 be controlled at some higherpressure in order to maintain the glass level in the finer chamber 24.One advantage of using a standpipe is that the bubbles are removed fromthe molten glass at multiple points, accomplishing more separation.Another advantage is that the foam collected in the standpipe 102 can besubjected to more severe foam breaking measures than can be carried outin the finer chamber 24. The methods described above may be used tocontrol the foam in the finer chamber 24.

[0031] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate thatother embodiments can be devised which do not depart from the scope ofthe invention as disclosed herein. Accordingly, the scope of theinvention should be limited only by the attached claims.

What is claimed is:
 1. A method for controlling the foam produced when amolten material encounters reduced pressure in a vacuum chamber,comprising: passing the molten material through an aging zone in thevacuum chamber, wherein the molten material is allowed to drain frombetween the bubbles of the foam; and breaking the bubbles of the drainedfoam.
 2. The method of claim 1, wherein breaking the bubbles of thedrained foam includes using a mechanical rotator to stretch and rupturethe bubbles.
 3. The method of claim 1, wherein breaking the bubbles ofthe drained foam includes impinging a jet stream on the drained foam. 4.The method of claim 1, wherein breaking the bubbles of the drained foamincludes spraying a stream containing particulate materials on thedrained foam.
 5. A method for controlling the foam produced when amolten material encounters reduced pressure in a vacuum chamber,comprising: allowing the foam to flow from the vacuum chamber into afoam chamber separate from the vacuum chamber.
 6. The method of claim 5,wherein the foam chamber is maintained at the same pressure as thevacuum chamber.
 7. The method of claim 6, further comprising allowingthe molten material between the bubbles of the foam to drain.
 8. Themethod of claim 7, further comprising breaking the bubbles of thedrained foam.
 9. The method of claim 5, further comprising cooling andcrushing the foam.
 10. A method for controlling the foam produced when amolten material encounters reduced pressure in a vacuum chamber,comprising: allowing the molten material to flow into the vacuum chamberthrough an inclined conduit, wherein bubbles in the molten materialcollect at a surface of the conduit.
 11. The method of claim 10, furthercomprising removing the bubbles which collect at the surface of theconduit through a standpipe on the conduit.
 12. A reduced pressurefiner, comprising: a finer chamber having a pressure below atmosphericpressure; a first conduit for conducting molten material to the finerchamber, the first conduit being inclined to the vertical, wherein thebubbles in the molten material collect at a surface of the first conduitas the molten material flows through the first conduit; and a secondconduit for removing the molten material from the finer chamber.
 13. Thereduced pressure finer of claim 12, further comprising a standpipehydraulically connected to the first conduit and through which thebubbles collected on the surface of the first conduit are removed. 14.The reduced pressure finer of claim 13, wherein the standpipe ispositioned such that the level of molten material in the standpipe isabove the level of molten material in the finer chamber.